Land-based foraging by polar bears reveals sexual conflict outside mating season

According to sexual selection theory, the sexes are faced with opposing evolutionary goals. Male fitness benefits from access to females, whereas female fitness is constrained by food resources and safety for themselves and their offspring. Particularly in large solitary carnivores, such as polar bears (Ursus maritimus), these divergent goals can potentially lead to conflict between the sexes. Outside the mating season, when polar bears are on the move across vast distances, the consequences of such conflict can become apparent when individuals arrive at the same food source. To investigate interrelationships between the sexes, we observed successive polar bears visiting a bird breeding colony to feed on clutches of eggs. We found that males succeeded females more frequently and more closely than expected by chance. Moreover, when males were closer to conspecifics, they walked faster, spent less time in the colony and ingested less food. In contrast, female foraging performance was not associated with proximity to other bears. Irrespective of proximity, females generally spent short periods in the colony and ingested fewer clutches than males. Our results suggest that in polar bears, there is a trade-off between the benefits of food intake and the opportunities (in males) and risks (in females) posed by encountering conspecifics.

Table S2.Candidate set of models for glmm models (R function glmmTMB adopting a binomial distribution) examining variation in the probability that the focal polar bear is a male.Fixed factors were age (Age) of the focal animal, and sex (PrevSex) and reproductive class (male, female without cubs, female with cubs) of the previous individual (PrevRepro).Covariate was the number of days between successive individuals (Interval).Random intercepts included individual identity nested within year of observation.Given for each model are the number of parameters (K), log-likelihood, AICc corrected for small sample sizes, difference in AICc relative to the model with the lowest AICc (Delta), model probability (Weight), and the marginal (R2marg) and conditional (R2cond) coefficient of determination.Interactions are indicated by '×'.The models are ordered by increasing AICc.Only the first ten top-ranking models are shown.Weight is based on the complete list of candidate models.The fixed part of the global model contained the following terms: PrevRepro×Interval×Age + PrevSex×Interval×Age.

Table S3 .
Selection results for glmm models (R function glmmTMB) examining variation in the seconds for 15 step cycles by polar bear while travelling in the core study area.Fixed factors were age (Age) and sex (Sex) of the focal individual, and sex of the previous individual (PrevSex).Covariate was the number of days between successive individuals (Interval).Random effect in all models was observation record nested within year.Given for each model are the number of parameters (K), Log-likelihood, AICc corrected for small sample sizes, difference in AICc relative to the model with the lowest AICc (Delta), model probability (Weight), and the marginal (R2marg) and conditional (R2cond) coefficient of determination.Interactions are indicated by '×'.The models are ordered by increasing AICc.Only the first ten top-ranking models are shown.Weight is based on the complete list of candidate models.The fixed part of the global model contained the following terms: PrevSex×Sex + Interval×Sex + Age×Sex.

Table S4 .
Selection results for glmm models (R function glmmTMB) examining variation in the

Table S5 .
Selection results for glmm models (R function glmmTMB) examining variation in number of clutches taken by polar bear in the study bird colony.Fixed and random effects, and zero-inflation part as in supplementary TableS4.Given for each model are the number of parameters (K), Log-likelihood, AICc corrected for small sample sizes, difference in AICc relative to the model with the lowest AICc (Delta), model probability (Weight), and the marginal (R2marg) and conditional (R2cond) coefficient of determination.Interactions are indicated by '×'.The models are ordered by increasing AICc.Only the first ten top-ranking models are shown.Weight is based on the complete list of candidate models.The fixed part of the global model contained the following terms: PrevSex×Sex + PrevRepro×Sex + Interval×Sex + Density×Sex + Age×Sex + Zi(Density).

Table S6 .
Selection results for glmm models (R function glmmTMB) examining variation in the probability of polar bears taking a rest in the study bird colony and adjacent tundra.Fixed and random effects as in supplementary TableS4.Given for each model are the number of parameters (K), Log-likelihood, AICc corrected for small sample sizes, difference in AICc relative to the model with the lowest AICc (Delta), model probability (Weight), and the marginal (R2marg) and conditional (R2cond) coefficient of determination.Interactions are indicated by '×'.The models are ordered by increasing AICc.Only the first ten top-ranking models are shown.Weight is based on the complete list of candidate models.The fixed part of the global model contained the following terms: PrevSex×Sex + PrevRepro×Sex + Interval×Sex + Density×Sex + Age×Sex.Data Records.Records of polar bears, in core study siteNordenskiöldkysten, 2009Nordenskiöldkysten,  -2022.Details are given for each of the records.